Field of the Invention
The present invention relates to a liquid ejection head and a liquid ejection apparatus for ejecting a liquid.
Description of the Related Art
Some liquid ejection heads for ejecting a liquid, such as an ink, include a recording element substrate and an ejection orifice member. The recording element substrate includes a plurality of energy generating elements for generating thermal energy used to eject a liquid. The ejection orifice member has ejection orifices from which the liquid is ejected. Heat generating resistor layers, which generate heat when electrified, are used as the energy generating elements. The heat generating resistor layers generate heat for forming bubbles in the liquid, and the liquid is ejected from the ejection orifices due to pressure generated by the bubbles.
The energy generating elements are covered with an insulation film, and a protective film is formed on the insulating film. The protective film is made of an electroconductive material such as tantalum (Ta). The protective film serves to protect the energy generating elements from a so-called cavitation impact and from a chemical reaction that occurs in the liquid. Here, the term “cavitation impact” refers to an impact that occurs when bubbles, generated by the energy generating elements, burst.
There is a concern that, if the insulation film of the liquid ejection head has a defect, such a hole (pin hole), the energy generating elements and the protective film may be electrically connected to each other, so that the protective film and the liquid may cause an electrochemical reaction, leading to degradation of the protective film. When degradation of the protective film occurs, the thermal efficiency of energy transferred from the energy generating element to the liquid changes. Therefore, for example, it is necessary to check the electrical insulation between the energy generating elements and the protective film during the process of manufacturing the recording element substrate.
Japanese Patent Laid-Open No. 2000-280477 discloses an inkjet recording head including a protective film and a circuit for detecting whether or not en electric current flows through a protective film and a liquid (ink) in a flow path.
Such detection can be easily performed by connecting protective films in the same recording element substrate so as to be electrically connected to each other and by using a terminal provided in the substrate. On the other hand, when the protective films are electrically connected to each other in the recording element substrate, degradation of some of the protective films may affect all the protective films in the substrate.
Some liquid ejection heads, such as a full-line head having a length corresponding to the width of a recording medium, includes a plurality of recording element substrates. If the substrates described above, in each of which the protective films are electrically connected to each other, are used for a liquid ejection head of this type, the following problem may occur.
If the liquid ejection head is assembled by using a plurality of substrates including a substrate in which a protective film has a defect, the thermal efficiency of the protective film having a defect deviates from those of other substrates, and therefore the minimum energy required for ejecting a liquid varies. When the liquid ejection characteristics of some of the substrates vary as described above in a liquid ejection head having a plurality of substrates, such as a full-line head, the amount of ejected ink varies and therefore the density of recording becomes nonuniform.
The same problem may occur if a hole is formed in the protective films or the insulation films of some of the substrates during use of the liquid ejection head.